The present invention relates to a device for fixing an internal structure in a removable or detachable manner to a wall having a circular cross-section, formed within an envelope, such as a pipe or a container.
Such a device more particularly applies to fixing certain structures within the primary circuit of a pressurized water reactor. However, this application is not limitative and the device according to the invention can be used in all cases where it is necessary to fix in a detachable manner a structure within a wall having a circular cross-section without damaging the latter and without there being any clearance between the internal structure and the wall to which it is fixed.
In the particular case of fixing internal structures in enclosures and pipes of the primary circuit of pressurized water nuclear reactors, said fixture must comply with conditions which are very difficult to fulfil. Thus, the fixing of internal structures must not prejudice the security or safety of the reactor pressure envelope and must make it possible to permit the inspection of the envelope in service via the interior of the primary circuit. As a result of the latter constraint, the internal structures must be dismantlable and there must be no reliefs on the inner wall of the pressure envelope. Moreover, due to the hydrodynamic forces produced by the outflow of the coolant, the internal structures must be secured without any clearance in order to prevent vibrations. Moreover, in order not to increase the pressure drop of the circuit, the device for fixing the internal structures must have minimum overall dimensions. Finally, it must be possible to dismantle the internal structures in a remote manner in the most radioactive parts of the circuit, which makes it impossible to use complex mechanisms.
In the present state of the art, the fixing of certain internal structures of nuclear reactors is sometimes brought about by heat shrinkage. This fixing procedure consists of using the temperature rise of a circuit in order to take advantage of a thermal expansion of the internal structure, which is greater than that of the pressure envelope. Thus, for the two structures, use is made of materials having different heat expansion coefficients.
This conventional procedure suffers from numerous disadvantages. Firstly the differential heat expansion between the materials forming the two structures is limited, so that very precise machining operations are required. Moreover, it leads to differential longitudinal expansions, which may be the cause of various hazards. It can lead to high stress levels within the pressure envelope. Finally, the parts are machined in such a way as to be fixed at the normal operating temperature of the reactor, which implies that the internal structures are secured with a clearance in the cold state, so that there is a high vibration risk in this state.